1. Field of the Invention
The present invention relates to a DC-DC converter formed of a boost chopper circuit, and particularly relates to a DC-DC converter employed in an electric automobile.
2. Description of the Prior Art
FIG. 1 is a circuit configuration diagram of a conventional DC-DC converter. The DC-DC converter is described in Japanese Patent Application Publication No. 2006-262601. The boost DC-DC converter has a direct current power supply Vdc1, transformers T3 and T4, a reactor L3, switches Q1 and Q2, diodes D3 and D4, a smoothing capacitor C1, and a control circuit 100.
The transformer T3 has a primary winding 5a (with np turns), an additional winding 5b (with np1 turns) connected to the primary winding 5a in series, and a secondary winding 5c (with ns turns) electromagnetically coupled to the primary winding 5a and the additional winding 5b. The transformer T4 is configured to be identical to the transformer T3, and has a primary winding 6a (with np turns), a winding 6b (with np1 turns) connected to the primary winding 6a in series, and a secondary winding 6c (with ns turns) electromagnetically coupled to the primary winding 6a and the winding 6b. 
The drain and source of the switch Q1 formed of a MOSFET or the like is connected respectively to both ends of the direct current power supply Vdc1 via the primary winding 5a of the transformer T3. The drain and source of the switch Q2 formed of a MOSFET or the like is connected respectively to both ends of the direct current power supply Vdc1 via the primary winding 6a of the transformer T4. A first series circuit is formed of the additional winding 5b of the transformer T3, the diode D3, and the smoothing capacitor C1. The first series circuit is connected to the connection point of the primary winding 5a of the transformer T3 and the drain of the switch Q1 and to the source of the switch Q1. A second series circuit is formed of the winding 6b of the transformer T4, the diode D4, and the smoothing capacitor C1. The second series circuit is connected to the connection point of the primary winding 6a of the transformer T4 and the drain of the switch Q2 and to the source of the switch Q2.
The reactor L3 is connected to both ends of a series circuit of the secondary winding 5c of the transformer T3 and the secondary winding 6c of the transformer T4. The control circuit 100 turns on/off the switch Q1 and the switch Q2 with a phase difference of 180° based on an output voltage Vo of the smoothing capacitor C1.
In the conventional DC-DC converter configured in this manner, when the switch Q1 is turned on by a Q1 control signal Q1g from the control circuit 100, the current flows from the plus side of the direct current power supply Vdc1, to the primary winding 5a, to the switch Q1, and then to the minus side of the direct current power supply Vdc1. Therefore, a current Q1i of the switch Q1 increases linearly. Simultaneously, voltage also occurs in the secondary winding 5c of the transformer T3, and a current L3i flows from the secondary winding 5c, to the reactor L3, to the secondary winding 6c, and then to the secondary winding 5c. 
The current L3i flows according to the law of equal ampere-turns of the transformer. Here, energy is accumulated in the reactor L3, and the same current flows also in the secondary winding 6c of the transformer T4. Therefore, voltages are induced in accordance with the number of turns in the primary winding 6a and the winding 6b of the transformer T4.
When a turn ratio in the transformer T4 is A=(np+np1)/np, a current of 1/A of the current Q1i of the switch Q1 flows from the plus side of the direct current power supply Vdc1, to the primary winding 6a, to the winding 6b, to the diode D4, to the smoothing capacitor C1, and then to the minus side of the direct current power supply Vdc1, thereby flowing in the diode D4. A current D4i of the diode D4 flows until a time at which the switch Q2 is turned on. The output voltage Vo of the smoothing capacitor C1 is a sum of the voltage of the direct current power supply Vdc1 (input voltage), the voltage generated in the primary winding 6a of the transformer T4, and the voltage generated in the winding 6b of the transformer T4.
When the duty cycle of the switch Q1 is D (D=Ton/T), the voltage generated in the transformer T4 equals A·Vdc1·D. Ton indicates the time for which the switch Q1 is turned on. T indicates the switching cycle of the switch Q1. The output voltage Vo of the smoothing capacitor C1 is expressed by Vo=Vdc1 (1+A·D), and the output voltage Vo can be controlled by varying the duty cycle D.
Next, the switch Q1 is turned off by the Q1 control signal Q1g from the control circuit 100. At this time, a current D3i flows from the plus side of the direct current power supply Vdc1, to the primary winding 5a, to the additional winding 5b, to the diode D3, to the smoothing capacitor C1, and then to the minus side of the direct current power supply Vdc1.
Next, the switch Q2 is turned on by a Q2 control signal Q2g from the control circuit 100. At this time, the current flows from the plus side of the direct current power supply Vdc1, to the primary winding 6a, to the switch Q2, and then to the minus side of the direct current power supply Vdc1. Therefore, a current Q2i of the switch Q2 increases linearly. Simultaneously, voltage also occurs in the secondary winding 6c of the transformer T4, and the current L3i flows within the reactor L3 from the secondary winding 6c, to the secondary winding 5c, to the reactor L3, and then to the secondary winding 6c while increasing.
The current L3i flows according to the law of equal ampere-turns of the transformer and the like. Here, energy is accumulated in the reactor L3, and the same current flows also in the secondary winding 5c of the transformer T3. Therefore, voltages are induced in accordance with the number of turns in the primary winding 5a and the additional winding 5b of the transformer T3.
When the turn ratio in the transformer T3 is A=(np+np1)/np, the current Q2i of the switch Q2 which is 1/A flows from the plus side of the direct current power supply Vdc1, to the primary winding 5a, to the additional winding 5b, to the diode D3, to the smoothing capacitor C1, and then to the minus side of the direct current power supply Vdc1, thereby flowing in the diode D3. The current D3i of the diode D3 flows until a time at which the switch Q1 is turned on. The output voltage Vo of the smoothing capacitor C1 is a sum of the voltage of the direct current power supply Vdc1 (input voltage), the voltage generated in the primary winding 5a of the transformer T3, and the voltage generated in the additional winding 5b of the transformer T3.
In this manner, in the multiphase boost chopper circuit with transformers shown in FIG. 1, two independent phases are combined by transformers. With such configuration, only one core is necessary instead of two cores, and a boost operation can be performed by the one core.
However, in the DC-DC converter shown in FIG. 1, recovery losses occur in the diodes D3 and D4. Also, switching losses occur when the switches Q1 and Q2 are turned on.